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For an electric airplane are propellers the only option for propulsion?
This is for an electric airplane (so no fuels for combustion):

Are propellers the only option?
Is there a turbine-jet possible with only air intake, electrically compress, and  a turbine and nozzle and get equal or more thrust than a propeller like a water-jet but then with air?

You might like to state what sort of altitude and performance you're thinking of.  A slow plane intended for use up to say 3-5000 ft will probably be more efficient with a propeller, very high altitude like above 30000 ft might be more efficient with a higher exhaust/nozzle speed, like your option "2".

Propellers are not the only option. I'm thinking of a ducted fan.

If you need super flight, you would need to use a jet, e.g. use intake to slow the flow to subsonic, compress it further, and jet it out as a supersonic flow. You couldn't do it propeller or even ducted propeller because ducted not not they don't run well with fast flow (even high sub sonic) and at fast speed.

Propellers in some form are really the only option. It is currently the only efficient way to generate thrust with air as a medium. Even jet engines are in some sense just a very elaborate system of propellers.
The reason for the turbine is to create an environment to efficiently burn fuel (suck, squeeze, bang, blow) to generate as much thrust as possible. Hence, in an electric aircraft this is irrelevant. You might be interested in the Airbus E-Fan, which uses ducted propellers increase efficiency. However, it is still 'just' a propeller with an electric motor in the middle.

It's not about efficiently burning fuel, it's about efficiently converting heat to work. For more info, check the Wikipedia article on the Brayton cycle.

You may choose whatever propulsion you prefer, but need to accept the consequences. An electrical turbine is easy to build, but will need a lot of power for heating the core flow. Instead of burners you will need to install heater elements, which will necessitate a longer flow path and more volume between compressor and turbine, but by and large the electric turbine engine will look much like a fuel-burning one.
You can also remove all the turbo machinery and only drive the fan with a big electric motor. But that would have lower efficiency than turning a propeller instead, so only when your flight speed will cause supersonic propeller tips might an electrically powered fan be a better alternative.
Therefore, at speeds below Mach 0.7 propellers will give you more thrust per kW of power, and their efficiency is best at low speeds. The much lower energy density of electric storage will demand flight at low speed, or your flight time will be very short. Therefore, propellers are the best, but by no means the only solution.
A potentially even more efficient method might be ionic thrusters where a high voltage is used to ionize and accelerate air in order to produce thrust. Designs using ionic thrusters would use the whole airframe for thrust creation and would need to be run at tens of thousands of Volts, but their potential efficiency surpasses even that of low-speed propellers - in theory. The linked article, however, is an embarrassment to MIT. A 1 kW jet engine that produces only 2 N thrust has to fly very fast (500 m/s, to be precise) while the 110 N cited for the ionic thruster at the same power mean that it could not had moved faster than 9 m/s.

Wait are you really suggesting to use heating elements to make an electric turbine? I get that just speaking of an "electric turbine" is quite a leap but turning electricty into heat is probably the worst thing you can do with that.

@VladimirCravero: He didn't say it would be a good idea.

@FredLarson Yep but that's like saying that gas is inefficient to propel boats because if you throw it off the stern the ship barely moves

@VladimirCravero: A turbine extracts energy from fluid flow.  Other kinds of devices like impellers and fans are used to convert energy into fluid flow.  Using electric heat to drive a turbine jet would be silly, but fundamentally the purpose of a turbine jet is to convert expansion of gases into thrust, and there aren't a whole lot of ways to make most kinds of gas expand electrically other than by heating.

I know currently electric density is an issue however as time progresses new technology will overcome this issue.
One off the ideas is to take off electrically for area/sound reasons and then switch to combustion engine which will drive electric systems/propellor/jet.

Looking at boats you have propellor and waterjet. A waterjet basically sucks water propels inside and pushes it through a smaller nozzle.
My questions was would this also work for air - thus suck air propel inside and push it through a smaller nozzle for more speed as propellors max out around 350 mph. The main thing is it works electrically.

@Arturo Water is different - it is a liquid and (nearly) incomprehensible.  You want an electric plane that goes over 350 mph?

The idea is a hybrid plane with combustion + battery. Which takes off on battery for low sound and then switch to a combustion engine which will convert to electricity to drive the electrical propellors / waterjet / car wheels or use the electrical energy on ground as a generator in emergency. For flying speeds from 350 mph to 550 mph (if possible) would be ideal.

This would not be done by battery alone but switched to the normal engine. During flight the batteries would be refueled for battery landing procedure (low sound).

I know the status of current batteries however new battery inventions will 4x densify with a 5 minute recharge time will become available in the coming 3-10 years. (eg a Tesla with theoretical 2.008 km range and recharge in 5 min)

I think this answer is essentially correct, but I should mention that "heater elements" are probably not the best option. The nuclear jets tested used heat exchangers to get heat from the reactor to the air, I think. This would probably be better for electric as well. You can cause an electric discharge over on air gap so that heat is generated in the air. Or microwaves at frequency absorbed by nitrogen. Or heat an inert fluid electrically, and use that to drive the turbine. But I totally agree propellers are the way to go here. Just add a gas turbine with generator to make it into a hybrid.

Ionic Thrusters! Now there's a marketable phrase!

@Koyovis: Yeah, I found out about them yesterday and thought they should be mentioned here. But the more I research them, the more they sound like quackery. The thrust comparison with jets is total nonsense, and ionization is wasteful, too, producing ozone and nitrogen oxides.

@PeterKämpf Is ~900km/h theoretically possible w. an electric motor, energy output considerations aside ? Would this be achieved using a ducted fan despite the probably supersonic propeller wingtips ?

@Stanislasdrg: Yes, if you have enough battery capacity to accelerate this fast. Also, you should do that at high temperature and high altitude, so density and Mach are low. I would prefer slow-moving, large propellers. See here for more.

Ducted fans rather than external propellers increase the performance at high speeds, so gets round most of the problems.
You aren't really going to want to carry fuel AND batteries, so some sort of ducted fan is the best solution for a high speed aircraft.
While they do look somewhat like Jet engines, they are basically just prop engines in a tube, so they can better control the air-flow, which at high velocities, can cause issues for exposed props.

That does raise the interesting question, can you use a ducted fan rated for cruise power, and inject and burn fuel into the compressed airflow (like reheat or afterburner) for takeoff power? Yeah the compression ratio is probably too low...

No, they are nothing like jet engines.

@Drenzul: He didn't say that they were like jet engines - just that, from the outside, they look kind of like jet engines (due to the big round cowling duct).

Suggesting that they can use a similar afterburner system as Jet engines would imply he thought there was more similarity between them than there is, probably because from the outside they do look similar.

You do not have hot combustion gases in an electrically powered engine. The only gas available to you is the air around you, and some form of propeller is the most logical way to accelerate that air.
The nozzle you're imagining can be used to turn a large, slow stream of air into a faster but smaller air stream. This can't provide thrust by itself as it isn't a powered engine part.

The electrically-powered compressor is the "powered engine part" in the second scheme in the question.

@DavidRicherby: Indeed. But the scheme presented there (with a turbine) doesn't really make sense. The compressor is already electrically powered and doesn't need to be powered by the turbine. Nor would the turbine be able to extract energy from the combustion gases.

For your requirement to use a battery for silent takeoff and landing, electrically driven propellors (or ducted fans) would certainly be good enough.
The requirement you mentioned for 350 mph to 550 mph is really only for cruise, which presumably would use a turbofan (or possibly turboprop) - and recharge the battery at the same time.
So I would guess what you want is a fairly high performance ducted fan which can be driven by either electric motor for silence or by a conventional turbine at cruise - a bit like a turboprop.  The question is then how do you arrange for an exchange between the two power sources.  (I assume two sets of engines wouldn't be possible, though perhaps they could be?)
(Edited to add... your electric motors still need full power to takeoff.  So why not run the motor all of the time, and use an auxiliary turbine engine to provide the electrical power at cruise?  Not unlike a hybrid car.)

Yes, two sets of engine. 1. Battery for silent takeoff. 2. Combustion engine which transfers to electricity which then drives the electronic propellor.

In other words - something like a more powerful version of the E-Fan (which MikeFoxtrot linked to) with an auxiliary turbine to generate electricity at cruise.  (Provided suitable motors can be found, and a ducted fan design which can provide the necessary performance both at climb and cruise.)

Keep in mind, much of the noise for prop drive aircraft is the propeller itself.  Kind of like how an electric fan makes whole bunches of noise, even though it's electric.

Relying on an electric power source, one COULD use a winch or linear motor for the on-the-ground phase. Even a bit longer. While at it, the power for the on-ground phase can even be sourced externally, saving battery power for the climb and cruise phases.
With externally source power from the ground, and motors on board, we might explore MAXIMUM take-off speed. Props don't like getting to Mach 1, I get it. So let's just keept those folded away for take-off. With a lineair motor in the runway, and ideally gradually pointing up by the end, we might take-off well over Mach 1. And with that linear motor dragging us up to speed, and into some good vertical speed before even really taking off, we might as well put some good water tanks on board.
Why? The mass add kinetic energy and that'd GOOD to no sow down too much as we encounter air drag at high speeds in a low powered prop plane, let alone with the props tucked away. We can drop the water ballast as we approach the break even speed/altitude.
But wait, now that we take on wair as ballast, we might as well make it a bit of water rocket. We super heat it to high pressure, which offers huge thrust for as long as the water is being dumped overboard. After the catacult-off, when we reach the appropriate speed for the water rocket to be efficient, we start the rocket phase and this may well overlap with the first prop-drive phase. We climb REALLY fast until we run out of water and then ease into a very casual cruise.
The range on the range battery is now a lot longer, assuming the pressure vessel isn't too heavy.
Oh, and for the whole take-off phase, we kept the landing gear tucked away. We weren't landing, after all. The linear motor provided support to the ground.
I'll save everyone my thoughts on hubless/coreless props.

This reads somewhat as an extended comment, and would benefit from and edit to summarise the three concepts (catapult launch, water ballast and steam rocket) along with some paragraph structure. Some math on how much height a mach1 launch might get a light aircraft, along with https://en.wikipedia.org/wiki/Steam_rocket would also strengthen things.

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center.

The main difference between air and water is that the second cannot be compressed, while the first can and when you do, you heat it up loosing a cospicuous amount of energy. The more you compress it and the more time you wait before releasing it, the more energy you loose.
A propeller also compresses the air: this is necessary to accelerate it, but it allows to achieve high air velocities with the minimum amount of compression. It is also a very simple and light design, moreover one of the motivation to have an internal water compressor is the possibility to filter it, which is not required with air.


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What kind of balloons are these?
I came across the following picture on a news site, in a series of 'this day in...' type items.

The accompanying text reads (in Dutch)

22 juni 1975 - In Geldrop werd de Internationale Holland Ballonwedstrijd gehouden. Deelnemers uit zeven landen streden daar om de Trophee du ballon libre.

Which translates to

22 June 1975 - The International Holland Balloon competition was held in Geldrop. Contestants from seven countries competed for the Trophee du ballon libre.

To my surprise, the balloons are round and are closed at the bottom. What type of balloons are these? They don't seem to be hot air balloons, are they?

helium or hydrogen I'd guess

@ratchetfreak That was my thought too, but I'd like something more definitive, preferably with more of an explanation, how common is / used to be this type of balloon and so on.

Assuming you are Dutch, I think you will appreciate this little gem

@DeltaLima Yes I am, and thank you.

These are gas balloons. Instead of heating the air to reduce its density and providing the buoyancy, these balloons use helium or hydrogen which is lighter than air. Before lifting off, the balloons are kept on the ground by extra weight hanging on the basket. Usually these bags, clearly visible on the picture, are filled with sand.
During the flights, the gas will slowly dissipate through the fabric of the balloon reducing the buoyancy of the balloon. By throwing more weight overboard, again sand, the balloon can maintain in the air.
Gas balloons have less control in height than hot air balloons. The balloons are more expensive to operate because of the price of helium and to a lesser extent hydrogen.
Although gas ballooning is less popular then hot air ballooning, there are a number of gas balloons operating in Europe and the USA.

Once a year the Gordon Bennett race  is held in which teams compete to fly the longest distance in their gas balloon.

After some more research into the Trophee du ballon libre I found this Polygoon Journaal item of 1976 (in Dutch). It mentions that the balloon is kept afloat by 630 cubic meters hydrogen gas.

I thought as much, but I expected to see some of the gas bottles still lying around. Was it common to use helium instead of hot air in the 70's?

I wish I could vote your answer up more than once. You went above and beyond. (Pun intended only if you want to).

For the worse control and higher operating cost gas filled balloons can lift more payload or take the same payload higher for the same size, because density of hydrogen is much lower than that of even very hot air.

If one travels to Paris (but I'm sure there are other locations), you may board one which is a permanent attraction (and also used for air analysis, and unfortunately for advertising too), near the Eiffel tower. It uses helium and is able to lift 2.5t at 150m, according to Wikipedia.

@SQB That's very kind of you to award me a bonus! It was really not necessary for you to do that, but I appreciate it!

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Advantages of square over circular engine nozzle?
The F-22 has square nozzles, while the Russian Sukhoi PAK FA has circular nozzles.
There are both 5th generation planes, but with different nozzles, why?

have you looked at this? http://aviation.stackexchange.com/q/16268/1467

@Federico It does not clarify the reason why Sukhoi has 3D-vectoring nozzles with 2 engines while Lockheed Martin decide to simplify.

that boils down to different specifications. And to know why the specifications look like they do, you have to ask that question to whom has written them.

Circular nozzles have these advantages:

They can be swiveled in any direction, so both pitch and yaw moments can be produced. Square nozzles only allow to create pitch forces, because their panels can only be moved around one axis. When moved differently in two-engined aircraft, limited roll moments are also possible.
Round nozzles are lighter. As long as exhaust gas pressure is higher than outside pressure, a square nozzle will be subject to bending loads across the width of its panels. This will make a square nozzle much heavier than a round nozzle.

Actually, square thrust vectoring nozzles have just one advantage: They can be made to reflect radar waves in a few well-defined directions, while round nozzles will scatter them all around. This makes square nozzles the preferred choice for stealth aircraft, regardless of thrust vectoring.

Engine nozzles of the F-22 (picture source). Note the serrated rear edges of the thrust vectoring panels.

Engine exhausts of the B-2. Since the B-2 uses four non-afterburing engines, fixed nozzles are used. Their location helps to protect the airplane from detection by infrared (IR) sensors, but makes thrust vectoring impracticable.

Engine nozzles of the YF-23 (picture source). The lower part is longer to shield the hot exhaust against IR sensors from below.
If the goal is to mostly reduce radar reflections in the forward hemisphere of the aircraft, round nozzles can be tolerated. Only when the aircraft needs to be protected from radar-guided missiles fired from behind will the square nozzles give a clear advantage. It seems that the people behind the Sukhoi PAK FA (and other stealth fighters like the Chinese J-20) do not worry so much about this detail.

Sukhoi T-50 (prototype for the PAK FA program) from below (picture by Dmitry Pichugin)

Chengdu J-20 rear view (picture source)
However, even Lockheed-Martin and Pratt & Whitney now place maneuverability and weight over stealth in their nozzle design for the F-135 engine of the F-35 strike aircraft. Note the serrated edges on all contour steps. This helps to reduce radar returns, but is much less effective than a square nozzle. The benefit is less weight and better maneuverability.

F-35 with the F-135 nozzle visible (Picture source).

Another reason the F-35 uses a round nozzle is because of the planned vertical thrust vectoring mode. The designers got the idea for its operation from an ordinary house ventilation elbow, which has angled rotating sections allowing a variety of configurations. The F135 engine variant in the B and C STOVL versions will rotate in a similar way, during flight, so the circular exhaust prevents unwanted thrust moments in odd directions during transition.

@KeithS: Uhhh - the Yak-141 is not a ventilation elbow, as far as I know. Actually, this technology was licensed from the Russians for the F-35. And to use a rectangular nozzle on the non-VTOL variants would have been easy.

Watch this Youtube, it shows the system in an actual F-35 airframe: https://www.youtube.com/watch?v=iRgcC9eqEJg. You can see the rotating sections in action as the nozzle transitions. The nozzle itself rotates through at least 45 degrees.

As far as the technology, yes it was Russian-derived, but less licensed and more outsourced. Lockheed basically financed Yakovlev's continued development of the nozzle after the CIS cancelled funding, in return for full access to Yak's work for use on the X-35.

As far as using a rectangular nozzle on the A-variant, the airframe was designed to be modular, allowing the STOVL systems to be incorporated into the same basic design as the CTOL A variant.. Using a different exhaust configuration on the A would have meant re-engineering the rear third of the aircraft, at least.

The type of nozzle used in the aircraft depends on the design decisions taken. In short, the designers of F-22 chose stealth over manuverability, while the Sukhoi PAK FA designers went the other way around.
Circular thrust vectoring nozzles have been used in Sukoi aircraft for quite some time. The main advantages and disadvantages of the two types of thrust vectoring nozzles are:

The circular thrust vectoring nozzles allow control in all the three axes. For example, the PAK FA's engines have a deflection of of 15 (or 16) degrees in pitch axis and 8 in yaw axis, and through differential movement, in roll as well.

Source: defencyclopedia.files.wordpress.com

The 'flat' thrust vectoring nozzles used in F-22 allows control only in the pitch (and roll) axis. However, they do allow higher deflection compared to the axisymmetric nozzles, with the F-119 engine used in F-22 allows deflections upto 20 degrees.

Source: pw.utc.com

Another point is that the PAK FA uses a variant of the engine used in Su-30, the AL-41F1. A new engine is still under development. So the final configuration can very well be different, though I doubt it as Russians seem to value maneuverability above stealth.
The main disadvantage of using a axisymmetric nozzle is that it is inherently unstealthy. As far as stealth goes, the circular (or spherical) section is one of the worst design choices. The F-22's nozzle design is optimized for stealth, especially  the edges of the thrust vectoring nozzles are carefully aligned to prevent radar reflection. This is clearly visible in the planform views of the two aircraft.

Source: http://manglermuldoon.blogspot.in

There have been efforts to reduce the RCS of axisymmetric nozzles by incorporating design choices like serrated edges, geometrical shaping (like very low curvature nozzle petals), advanced cooling system, and special coatings on internal and external structures. These were first tested on the F-16 Low Observable Axisymmetric Nozzle (LOAN) program and the resuls were applied to the F-35, another stealth aircraft with thrust vectoring nozzles.

Rear view of a standard P&W F-100-200 in F-16 and the LOAN nozzle in the foreground (LMTAS photo), Source: F-16.net
However, according to ausairpower.net analysis of J-20 which uses an engine similar to PAK-FA,

The tail aspect sector is largely degraded in RCS performance by the use of axi-symmetric nozzles which introduce strong specular and diffraction returns

In general, a square nozzle provides improved stealth behind and below the aircraft. The F-22 nozzle has serrated shaping similar to that in LOAN nozzles, which further help in reducing RCS.

Source: http://defence.pk

The nozzle being a rear orifice of sorts, the sentence containing the typo (just now corrected) had a rather rude second meaning (obviously unintended).  Sorry for pointing this out!

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What is a seal in aeronautics?
I've read phrases like "All gas turbine engines use pressurized seals for their shaft bearing compartments to keep [...]" but I do not understand what "seals" means. In spanish we haven't a word for it.

I think this is better on-topic at http://ell.stackexchange.com/

I'm voting to close this question as off-topic because it is a request for a dictionary definition which applies to almost any mechanical system so isn't really a question about aviation.

Although it request dictionary definition, it can be answered in aviation sense given OP gave enough context.

Of course there is a spanish word for it: junta. Seventh definition: http://dle.rae.es/?id=Md1HbkD

@DavidRicherby But you only know it isn't related specifically to aviation if  you know about aviation. It's entirely possible that aviation uses a different (or more likely, a specialized) definition of a seal that carries with it implicit knowledge of avionics. I'd be worried if the site was nothing but terminology questions, but I don't see anything wrong with the question itself.

Transalation to spanish is "sello" or "junta", I think is related to aviation as probably the question is not only translation, as includes as well understanding of the technology of the seals used. I recommend you to change the question a little bit.

The seal is the thing that launches the ball into the air.  A ball in flight falls within the scope of "aeronautics".

Translation
Seal means sello mecánico.
While the answer could stop here, we wouldn't know what seal types are used in turbine engines, and how some are pressurized to prevent contaminants entering the aircraft pressure vessel.

Need for bearing compartments and seals

I've read phrases like "All gas turbine engines use pressurized seals
  for their shaft bearing compartments..."

In turbine engines, there are one or more shafts holding the rotating parts of the compressors and turbines (rotors). Bearings are inserted between a shaft and its stationary supports to limit destructive and heat-generating friction. Bearings also bear the large axial load transferred by compressor and turbine rotors to the shafts.
In turbine engine bearings, solid friction is replaced by fluid friction using an oil film between balls or rollers and races. Bearings are enclosed in a cavity and oil is injected into the cavity. This cavity is known as a sump.
The following drawing is taken from  an answer to In a turbofan what holds the spinning axis?. You can see the five bearings within their sumps:

(Source)
Oil must be circulated from the oil tank to the sumps and back, and cooled before being reused. As oil quantity is limited, oil must be prevented from leaving the oil system. This is where seals enter into action. In particular, seals are used to prevent leakage between a rotating shaft and stationary sump apertures.
High speed seals
Turbo engines' oil seals are specially selected to bear the high rotation speed of the shafts (wear), and the high operating temperature of the engine core (adjustment and bending). They are of two main types: Carbon or labyrinth. Both reduce oil pressure while it travels in the interstice between the shaft and the sump wall.

Carbon (source) and labyrinth (source) seals
Air sealing

All gas turbine engines use pressurized seals

As one would think, these seals are not perfect: a small quantity of oil will be able to make its way across the seal, and this quantity will increase with seal wear. Unfortunately, burnt oil releases hazardous fumes which could enter the aircraft air conditioning system as cabin air is tapped most of the time into the engine compressor.
To limit the leakage further and to recover leaked oil, a second cavity is built around the first seal and pressurized air is injected into it. This has two effects:

Air pushes oil back into the sump.
The second cavity being also sealed, any leaked oil can be drained and dumped overboard.

For example, the forward sump (on the left of the first picture) encloses 3 bearings (from left to right: #1, #2 and #3). Oil must be contained within the sump near bearings #1 and #3. Let's look at a CFM56-7B engine mounted on Boeing 737 NG. This is the lower half of the forward sump:

CFM56-7B forward sump, based on drawing from CFM56-7B Familiarization Manual

Oil enters the sump and is directed to bearing races using tubes and oil nozzles.
An oil seal prevents oil from escaping the sump in large quantity.
As some oil would nevertheless cross the seal, compressed air is injected on the other side of the seal, to counter oil leakage. Air entering the sump is then dumped overboard by the centrifugal air-oil separators.
The space where air is injected is closed by a cap on the sump with a air seal on the shaft.
Air and a small quantity of oil able to cross the oil seal are eventually drained and released overboard.
Oil crossing the bearing is scavenged and returns to the oil tank after air has been removed.

Metal particles in scavenged oil are captured by magnets to detect abnormal wearing of bearings. From time to time oil is added to the oil tank to compensate for small leakages in the bearing sumps (and in the gearboxes).

See on Youtube: Carbon Seals - Turbine Engines: A Closer Look.

I find searching for "juntas de estanqueidad" on google gives me gaskets, not seals. Is there some other translation?

@Andy: Thanks for pointing out. I updated with sello mecánico (Google search).

Seals are things like this:

they prevent oils and other fluid from contaminating areas where they do not belong (and they prevent impurities from contaminating the oils).
You can see one of these used in this assembly diagram (the "oil seal") to protect the ball bearing assembly:

(Image source)

"Seal" is used here to mean "closure". Specifically, a seal is a mechanism which prevents gases (or liquids) from crossing a barrier.
For instance, a rotating shaft must be connected to a fixed structure by means of a bearing (which allows it to rotate). If dirt or liquid were to enter the bearings, it would damage the mechanisms inside. Therefore, seals are used to keep dirt and liquids out of the bearings. Here is an example from general machinery: http://www.machinerylubrication.com/Read/838/protect-bearings


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Where do military aircraft carry flares?
Many sources on the internet say flares are carried in the fuselage, but do not specify the exact location of the compartment.
Does anybody know exactly where they are stored?

The location of the flare/chaff dispenser (FCD) depends on the aircraft type and even in case of the same aircraft, they can be located in different place, depending on the version and customer.
For example, the F-16 carries the FCD in its body fairing, as can be seen in the following image of Hellenic Air Force F-16 Demo Team “Zeus”.

Image from theaviationist.com
The following image shows a close view of FCD.

Image from f-16.net, credited to Capt. Lance
The following image shows an Israeli F-16B 'Nezt' having FCD between its ventral fins.

Image from theaviationist.com

Initially, most of the FCDs were retrofitted to the existing aircraft and were simply attached to them. However, most of the modern aircraft have FCD bays integrated into their fuselage. Still, some aircraft can carry extra flares/chaffs in their pylons (called PIDS (Pylon Integrated Dispenser System)), as shown below in an RDAF aircraft.

Image from f-16.net, credited to Erik Frikke

Also, most of the combat aircraft have the FCD in the bottom, while transport aircraft (and helicopters) have them on their sides.

The A-10 has it on the underside of the wingtip. See photo on your answer to this question


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Why does this shockwave not touch the body that creates it?
In the shock wave of the next image (in a blunt body)

Why the wavefront is in front of the object? It does not assume that if the object travels faster than the speed of sound wave front should be just touching the object that causes it ?

@mins, edit of the comment? No. Well, the shock must be, always, ahead of the object. The shockwave touching the object would produce infinite pressure or some such physical impossibility.

Is this not adequately explained in the Wikipedia article where you found the image?

The air between the shock and the body also travels at supersonic speed. For the outside flow it looks like part of the body.

"In other words, when the needed rotation of the fluid exceeds the maximum achievable rotation angle for an oblique attached shock, the shock detaches from the body." - Nope; still not in English.

After passing through the shock wave, the air still has to evade the body. If the shock-wave touched the body, the air would have to escape through infinitely thin space at infinitely high pressure and it obviously can't do that.
So behind the shockwave there is a boundary layer (called shock layer) in which the air is moving with the body and flows, under high pressure, to the sides where it can escape.
The blunter the body, the thicker the boundary layer, because the air must cover longer distance to the sides and takes longer to do that.
Of course the high pressure between the shock and the body causes a lot of drag, which is why sharp nose and leading edges are preferred for supersonic flight.

Boundary layer sounds correct. A boundary layer is the layer between the bulk of a fluid (or gas) which isn't affected by a solid object and that object itself.

@MSalters, I still think there was a more specific term for distinguishing this kind of layer behind shock wave from the boundary layer around object seen in subsonic flow, but it's been long I read about this and I don't remember where.

You're thinking of the "bow shock". Indeed the image in the question comes from the Wikipedia article entitled "bow shock".

Entropy layer / shock layer?

@costrom, yes, that's it.

@LightnessRacesinOrbit, this kind of shock is indeed called “bow shock”, but I meant the subsonic layer behind it, which is called “shock layer”.

Fun fact: you can never really touch something, for basically the same infinite-force reasons. When you "touch" a desk, you're really just getting close enough that electromagnetism pushes the atoms in your hand and the atoms on the desk apart with a lot of force. When you stand on a rock, same thing. (You could probably say that you actually touch the ground if you're on a neutron star, but then you'd also be quite dead.)

In case of an infinestimally thin body in supersonic flow, the disturbances created propogate as Mach waves. However, as the the thickness of the body increases, the flow has to physically turn away from the object after the shock wave.
For example, if a wedge at an angle $\theta$ is placed in supersonic flow, the fluid has to turn by that angle after passing through the compression shock (so that the flow is along the body).
For a given upstream Mach number $M_{1}$, there is a maximum turning angle $\theta_{max}$, beyond which an oblique shock is physically not possible and the shock wave gets detached i.e. it is physically not possible to turn the flow suddenly while conserving mass, momentum and energy. The flow 'solves' this problem by detaching the shock in front of the body.

Perhaps, at the root of your question, there is a misunderstanding of the nature of a shock wave. A shock wave is a surface, an ensemble of points, where the properties of the gas change discontinuously (it is not the only surface with this characteristic, but the other class, that of contact discontinuities is absolutely unstable, and quickly disappears).
Thus a shock wave has gas on both sides of it, by definition. Hence, it cannot touch the object, because there must be another layer of gas between the object and the shock wave.
But now your question can become: why must there be gas on both sides of the shock wave? The reason is that fluid must cross the shock wave, i.e.  fluid goes continuously from the front to the back of the shock wave (discontinuous are the thermodynamic properties of the gas, and the component of the perpendicular speed across the shock wave, but otherwise gas move continuously from one side to the other).
Are there surfaces where matter does not cross a surface of discontinuity? Yes, there are, and they are called contact discontinuities, which, being absolutely unstable (the term absolutely has a very special meaning in fluidodynamics), quickly disappear.
The second part of your question,

It does not assume that if the object travels faster than the speed of sound wave front should be just touching the object that causes it ?

seems to indicate that you are thinking of a shock wave as a regular wave, hence the blunt object should catch up with it. Unfortunately, calling a shock wave a wave is a historical relic, which betrays the fact that shock waves move supersonically with respect ot the fluid in which they propagate. In fact, there can be no shock wave which moves sonically or slower, lest the second principle of thermodynamics be violated (Landau and Lifshitz's Fluid Mechanics volume has a beautiful proof of this theorem, in full generality).
As a matter of fact, we (could) take the above statements (in a quantitive form, mind you) and push them one step further: since the motion of the post-shock gas is slow (i.e., subsonic) with respect to both the shock wave and the blunt object, its flow away from the fore of the blunt object occurs slowly, so that more and more fluid is trapped between the shock wave and the object. More fluid means more counter-pressure, hence the the shock must be located further and further from the blunt object: it forms close to the tip of the blunt object, but then it drifts away from it.
The truly cute feature of the picture above is the well-defined turbulent region in the object's wake...
EDIT:
As per mins' request (see below), you can see it as follows.
The speed of sound arises when you consider the fate of small perturbations. Imagine for instance that you locally compress matter a tiny little bit, i.e. you increase the density from $\rho$ to $\rho + \delta\!\rho$. In fluids, a higher density of a given fluid (= a smaller occupied volume) induces an extra pressure, so that, locally, $p\rightarrow p + \delta\!p$. So now you have a small volume whose pressure exceeds that of the surrounding medium: what happens? It obviously expands, i.e. pushes away nearby matter in order to claim for itself the volume occupied by the surrounding medium. Hence, a small increase in local density results in nearby matter having to move away, dislodged by the extra-density volume.
The dislodged matter moves slowly, because the force acting on it is small: we assumed that the extra pressure is small compared to the average pressure, $\delta\!p \ll p$. However, the first dislodged parcel of fluid sits next to another (for the moment static) parcel of fluid, which itself must make room for the first parcel of fluid, giving rise to a second dislodged parcel of fluid, and so on. There will be $N \gg 1$ dislodged generations of parcels of fluids, all set in motion by the original compressed fluid element, and reached eventually by this chain of dislodgings.
At first sight, one might think that this chain of dislodging events goes only as fast as each parcel of fluid, which is very slow as we said above, but this is not the case. The reason is that what we are asking is not the speed at which is parcel of fluid is moving (slow, as stated above), but how long it takes for the $N$-th generation dislodging to begin, not to complete. The speed of sound identifies the moment when the $N$-th generation dislodging begins, not ends, it identifies when the first perturbation arrives, not when it completes.
This speed is much larger than the speed at which individual parcels move. When we have compressed the initial parcel, we have also heated it (compression leads to heating, in normal materials), hence the average molecule will have a higher-than-before temperature $T+\delta\!T$. As you clearly know, a molecule coming from a warmer region is slightly faster (in the average) than one from a colder region because $v \approx \sqrt{kT/m}$,
so that the first generation dislodged material will first notice that it has a warmer (=greater pressure) neighbor when it is first reached by its hotter-than-average molecules, which travel at speed $\approx \sqrt{kT/m}$. Thus the speed with which the message Slide over, you wimp travels is the same as that of the molecules from the warmer surroundings.
These hotter-than-average molecules from the hotter surrounding collide with the average local molecules, and transfer to them a bit of their extra energy, i.e. local molecules are heated by the collisions from the original place of compression. Now these newly heated molecules will move out to the region of the 2-nd generation dislodged gas, and repeat the process again: the speed with which the signal travels, i.e. the first perception that the local gas must start moving, is again (actually, always) that of the molecules, $\approx \sqrt{kT/m}$. Hence the sound of speed is $c_s \approx \sqrt{kT/m}$.
In the derivation above I have skipped some details, whose inclusion however only modifies $c_s$ to the extent of replacing the previous formula with $c_s = \sqrt{5 kT/2m}$. The real detail I left out is that, once compression occurs, in real fluids this does not necessarily lead to the maximum heating possible (which leads to the factor $5/2$ above), but it can be lower than that because some of the extra heat may be dissipated away before the warmer-than-their-surrounding molecules have managed to reach their surroundings. If heat is dissipated (via viscosity, radiation, whatever) then it takes longer for the first-generation fluid to realize that the initial parcel of fluid has been compressed, because fewer of the molecules arriving to it are warmer than its own molecules.

It would be perfect for my understanding if you could also revisit another relic: the definition of speed of sound: Is it the maximum speed an air molecule can travel by itself when pushed by its neighbors. If so why? Friction/viscosity?

@mins I have written a qualitative explanation of the speed of sound for the non-physicist in an Edit. I hope you find it satisfactory.


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Do we shake more if we are seated towards the tail rather than in the middle or the front?
Just like in buses on road, we feel more shaky when we sit in the back seats than the driver and other sitting in front. Is it the same for flights when we are in clouds?


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How can bird hazard be avoided?
How do airport authorities deal with with birds? Which procedures have acutually been used in order to minimize or even avoid birds on airside and landside? Is there any procedure that crew can perform in order to minimize the danger caused by birds?

Airports are responsible for bird control and should provide adequate wildlife control measures. Some major airports are provided with an ornithological department that has ravens or similar birds trained to drive away other birds from the airport premises.
However, this is not a 100% efficient solution and flight crews have procedures that must follow if birds are reported.
If large birds or flocks of birds are reported or observed near the runway, the flight crew should consider:

Delaying the takeoff or landing when fuel permits. Advise the tower and wait for airport action before continuing.
Take off or land on another runway that is free of bird activity, if available.

To prevent or reduce the consequences of a bird strike, the flight crew should:

Discuss bird strikes during takeoff and approach briefings when operating at airports with known or suspected bird activity.
Be extremely vigilant if birds are reported on final approach. If birds are expected on final approach, plan additional landing distance to account for the possibility of no thrust reverser use if a bird strike occurs.

References: http://www.birdstrike.org
http://www.boeing.com/commercial/aeromagazine/articles/2011_q3/4/

The airport authorities use a number of techniques to manage the hazard of bird strike in and near airports. These include,

Elimination of bird habitats- Birds usually come to the airport area due to the availability of food sources. Denial of food to birds by eliminating the sources (like landfills, garbage, crops, dumps etc) will reduce the chance of birdstirke by eliminating the source.
It also helps to keep the airport and surrounding areas unattractive to birds (for example, by eliminating water stagnation, vegetation cover etc) as it prevents the birds from nesting there.

Scaring them away- A number of airports use a number of bird repellents like pyrotechnics, cannons (mostly just prior to aircraft approach to prevent bird habituation), chemical repellents etc.
In some cases, specially trained animals (like falcons, dogs etc) have been used to scare away the birds or they are trapped.

In some cases, the flight schedule can be altered if bird movement is expected in the area (for example, if the airport is on a bird migration route).

The problem can be solved by simply killing the birds in the area, though this drastic step is usually not implemented except in the most extreme cases. In general, the nesting sites of birds are destroyed to prevent them from staying permanently near the airport.

Usually, a combination of above methods is used to manage the birds.
The only procedure available for flight crews is to either delay the flight (or land in alternate runway if its available) in case birds are detected over an area. This however, is dependent on a number of variables like fuel availability, flight schedule etc. There are some radar systems under development that could detect birds; however, these have not reached operational stage yet.


text:
What is this "flap-like" thing on the back of the F-15?
I saw an image of an F-15 landing. What is that flap-like thing in the top of the plane in this picture? I couldn't find it on any other pictures or drawings of this aircraft.

That's an air brake. The f-35 has one also and a few other jets have them too. Designed to slow the aircraft down even more with the addition of flaps.

It is a speed brake (air brake). Here is a closeup of it.

Image from aero-cafe.com
It is usually used reducing speed landing (approach). It is also used for other functions- to slow down quickly at high speeds and to prevent too much speed buildup in a dive.
This image shows it being used during approach.

"F-15A 21st TFW landing with extended speed brake 1984" by SSgt. K. Vail, USAF - U.S. DefenseImagery photo VIRIN: DF-ST-85-11671. Licensed under Public Domain via Wikimedia Commons.
Similar airbrakes are also found in other aircraft, notably the Su 27 and its derivatives.

Image from aerospaceweb.org

The F/A-18 and the F-35 have speedbrakes as well, to add a few more aircraft with those to the list.

@SMSvonderTann only the F/A-18 A-D has the speed brake, it was deleted for the E, F and G.

@SMSvonderTann I'm not sure about speedbrakes in F35. You're probably talking about the covers of vertical fan?

@aeroalias The A and C versions of the F-35 don't have a lift fan but the brake is still there, the B version does have a lift fan and the cover is used as a brake as well.

@SMSvonderTann hmmm...  My understanding is that the speed brakes of F35 operate by deflection of control surfaces.

@Moo Doesn't the F/A-18E Super Hornet has speed brakes too? The two things that deploy over the wings?

if they are over the wings, and they dump lift as well as create drag, they are called spoilers.

It's an air brake, also called a speedbrake.
https://en.wikipedia.org/wiki/Air_brake_(aeronautics)
Another view of an F-15 landing with the speed brake open:
Source: Wikipedia, click to embiggen


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Are elevator trim tabs required for an aircraft equipped with movable horizontal stabilizer?
The Bombardier C-Series aircraft has a movable horizontal stabilizer. It also has Trim Up/Down (discrete) switches in the cockpit.

Is the aircraft also equipped with trim tabs?
If so, when pilot adjusts the trim using the trim switches, which surface moves - horizontal stabilizer or trim tabs?
If an aircraft has both trim tabs and movable stabilizer, when are the trim tabs used for trimming?

Is the aircraft also equipped with trim tabs?

From the images available online (this is a youtube screenshot, click it to see the larger version), no. And see point 3 on why I was not expecting any different.

If an aircraft has both trim tabs and movable stabilizer

I do not see the need of such a solution, a trimmable stabilizer is already optimal, adding trim tabs would increase the complexity of the system with no significant benefits, as the use of tabs increases drag, something that a THS does not.
I do not know of any aircraft that uses both THS and tabs, but, should one exist, I presume one would use the tabs only if significantly faster than the THS actuation system, allowing the aircraft to be trimmed more rapidly, but afterwards the THS would take care of the trim allowing the tabs to go back to neutral and reducing drag.
This solution would be hardly beneficial, since commercial aircraft (as the one that prompted your question) do not preform manoeuvers that require such fast (and eventually frequent) re-trimming.
In addition, as GHB has pointed out in the comments, the elevator is already able to take care of fast transients, the trim tab would only provide unneeded complications (in the design, the maintenance, the control).

What would then be the difference between a fast trim tab and an elevator?

@GHB practically, not much, except the probable authority: the elevator would be able to provide much larger forces/moments. I've added it to the answer, thanks.

"...the trim tab would only provide unneeded complications (in the design, the maintenance, the control)."  One advantage I can see would be the ability to use the tabs as a secondary means of moving the elevator in the event of a complete hydraulic failure, like the B737 does.


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What are hydraulically powered and electrically controlled actuators?
I thought that the actuators are hydraulically controlled when the hydraulic fluid changes their position, but some aircraft (Bombardier, for example) has hydraulically powered and electrically controlled actuators.

What does electrically controlled means?
How does hydraulics power the actuator?

As wikipedia mentions, "Electro-Hydrostatic actuators" (EHA) are self contained, the only input they need is electrical power and commands.
The electricity powers up the hydraulic pump that is used to move the actuator piston, while the electrical commands are used to select what the actuator does through the use of a valve.

What does electrically controlled means?

Means that the position (and thus the action) of the valve is controlled via a solenoid. The following scheme shows a valve with two solenoids. The valve has three positions, indicated by the three squares. Depending on the electrical control signal, the current in the solenoids changes and thus moves the selector (the block with the three squares) laterally.

Image source

How does hydraulics power the actuator?

Through a pump. Where the fluid goes is selected by the electrically controled valve, but the piston of the actuator is moved by the hydraulic fluid.
In the image below, if the valve of the image above is in the central position, the actuator does not move.
If it is in the left position, the actuator retracts.
If it is in the right position (with the two arrows crossing) the actuator extends.

Image source

Wow. I stared at that first image for a while without a clue what it meant. Then I read through and got to the second image and it all became clear!


text:
Is the Runway Visual Range (RVR) communicated to the pilots?
During CAT IIIA/IIIB approach where the decision height is 50 ft/0 ft, if the Runway Visual Range requirement for the approach is not met, what is a pilot to do?
I mean, 50 ft (or even 100 ft?) seem so low enough  that a go around can not be initiated. Unless the pilot knows the RVR beforehand how can he be sure to perform CAT III approach?

They get the RVR via radio, they can get up to three different RVRs too, touchdown, mid-point and rollout.

Pilots get the RVR values either from weather reports or directly from ATC. RVR also shows up on ATIS reports when visibility drops to less than 1 mile. ATC will have a realtime readout of RVR and can communicate these values to pilots at any time by radio.
ATC RVR display

Source: http://mtechsystems.com
The CAT III approach should only be commenced if the ceiling and RVR values meet the minimum for the approach in use and for the current aircraft equipment status. Some airlines will have their own minimums which can differ from the published CAT III minimums.
Pilots will often ask for the latest ceiling and RVR values while doing the approach. ATC will also announce any changes or reduction in ceiling or RVR values while aircraft are on approach. If the new values are below limits, the crew should commence a go-around.
A go-around can be done from any altitude and the wheels can touch the runway during the go-around. Low atlitude, "Low Energy" go-arounds are practiced during re-current Simulator training.

So if I understand it correct a go around can be initiated even when the main landing gear touches ground?

Yes, most aircraft/airline procedures will say as long as reverse has not been selected, you may initiate a go-around.

Is RVR usually on the ATIS?

When would ceiling be controlling for a Cat III approach? Vis, yes, but ceiling???

Certain CAT III approaches require a decision height.  It can vary according to aircraft type, equipment status, specific airline, or specific airport.

Under Part 91 approaches, the pilot can "take a peak" meaning that she can fly the approach even if the reported ceiling and viz  are below minima. Part 121 cannot begin the approach unless reported WX is above minima


text:
Where can I find the transition altitude in different countries/regions?
Where can I find the transition altitude in different countries or regions?
Hoping to cover as many places as possible. Links to the applicable regulations would be nice.
See also:

What is the difference between "flight level" and "altitude"?

Where can I find regulations in different countries for flight level rules based on direction of flight?
EDIT: Instead of request a list, now the question asks for how to find in general. A few examples should still be given though.

It looks like you're looking for a list, which is a reason to close IIRC.

@raptortech97 it's also marked as community wiki, I'm not sure how that's handled.

@raptortech97 I believe lists are generally bad if they are subjective, but there is a finite and defined set of information requested here. I hope it will work but that can be up for discussion.

@fooot I thought remembered questions that asked for plain lists were not allowed, but I can't find any reference or evidence for that. And if there's no source out there already, the answer's helpful to the internet in general. I suppose the only concern is that there's no clear stopping point, and too many countries to find information for all of them. But I suppose people can just keep adding to a CW answer, so that seems reasonable. Hmm...

A Google search would be a good place to start with your research.

@RalphJ There are a lot of instances that mention various places, but usually not authoritative or extensive.

@RalphJ I could not find good references on this topic searching on Google. This list will probably not cover all countries, but can grow to be extensive.

I don't think it can, or should be not too broad. There appears to be no 'one list' findable on the Interwebz. We can make one. The more countries, the better. At least we're not duplicating wikipedia...

@CGCampbell I kinda agree, there should be a Community Wiki answer.

Asia

Hong Kong: 9,000 feet

Africa

South Africa: fixed and to be at least 1000 ft above the highest ground within 25nm radius of an airfield (AIC 20.2)

Europe

Belgium: 4,500 feet (AIP Part 2,, ENR 1.7.2)
Netherlands: 3,000 feet (IFR) / 3,500 feet (VFR) (AIP Part 2, ENR 1.7.1.1)

Oceania

Australia: 10,000 feet MSL (AIP ENR 1.7 (2.1.1))
New Zealand: 13,000 feet AMSL (CAA CAR 91.425)

North America

Canada: 18,000 feet (SOR/96-433 602.34)
United States: 18,000 feet MSL (14 CFR §91.121)
Mexico: 18500 feet MSL (ICAO SAM/IG/24-WP/2.5)

South America

Argentina: Depends on terminal airspace
Brasil: Depends on terminal airspace
Chile: Depends on terminal airspace
Colombia: 18,000 ft (SAM/IG/12-NE/23)
Ecuador: Depends on terminal airspace
Paraguay: 3000 ft (SAM/IG/12-NE/23)
Perú: Depends on terminal airspace
Uruguay: 3000 ft (SAM/IG/12-NE/23)
Venezuela: Depends on terminal airspace


text:
What factors lead to mounting engines towards the rear of aircraft
Some aircraft have engine beneath the wings and some have at the rear of the aircraft.
What are the factors that are considered while deciding where to mount an engine?
Is one configuration advantageous over other?

I am just a Flight Sim enthusiast so everything I say should be heavily scrutinized, but there is a flightsim that features the A-10C which has an engine configuration just like you describe as you can see in the picture.

The (ingame) flight manual has the following to say about the engine configuration:

Engines
  All versions of the A-10 have been powered by twin TF-34-GE-100A engines that are mounted high
  on the rear fuselage between the wings and the rear stabilizers. The unusual placement of the
  engines provides several distinct advantages:

The high mounting reduces the likelihood of the engines ingesting Foreign Object Debris
  (FOD) when operating from rough, forward bases in war-time.
Engines can remain running when aircraft is being rearmed and refueled. This leads to
  faster mission turn-around
Ease of servicing the engines
Reduced IR signature from below due to the shielding of the horizontal stabilizer.

Another related thing I found was how they combine with the leading edge slats, it says.

Added leading edge slats for improved airflow to engines at higher angles of attack

As most of these reasons are in a military setting I cannot comment on the advantages for civilian aviation.

This is just a pure copy-paste answer.

Got to admit, though, flight sim graphics are getting pretty good!


text:
Can a fighter jet be hit by guns?
Can a fighter jet travelling at fast speed be brought down by a weapon like an M134 Minigun or any machine gun?

related: http://aviation.stackexchange.com/q/13600/1467

I'm voting to close this question as off-topic because It attracted only answers with a military background. The question is specific to ballistics and weapons, the only connection with aviation is that it is "about" a fighter jet.

@BogdanWilli - Ehhhh.... AAA is a pretty important fixture of military aviation and has had a big impact on their design and how they're operated. You don't really have to know much about their ballistics to appreciate their effects on operations. If I have time, I'll edit in some more info on how AAA has affected a/c design.

If the bullet and the aircraft's trajectories are convergent, the bullet will hit the aircraft, whatever their velocities. Put the bullet in the adequate trajectory require some skills (and a good gunsight)

@HephaestusAetnaean: What is AAA? can you explain please? Your answer is quite a good answer indeed. But still I think the whole thing (especially your answer) would be better on skeptics or on another military or stragegy related website. See in the help center that also Rocket -related questions are off topic.

@BogdanWilli - Ah. Fair enough. I forgot the scope of Aviation.se; the F-16 and B-52 questions were fresh in my mind. ....... BTW, AAA = antiaircraft artillery (think WWII flak... sort of). It basically forces a/c to fly at higher altitudes so that they can't hide in the ground clutter and between hills/mountains etc. It spurred (in part) the development of better radars, stealth, better air-ground sensors, and modern multirole fighters like the F-35. And it made life very difficult for low-level a/c like the A-10, F-111, and B-1.

They're called SPAAG's -- Self Propelled Anti-Aircraft Guns. Older systems tended to fire 23 mm shells. Newer systems tend to fire 30 mm shells augmented by short range missiles.
They are equipped with radars and high-speed elevation and traverse motors for the turreted guns, so they can react quickly to fast a/c flying low ahead. They're also useful against attack helicopters, which also hug the ground (see below).
These are short range weapon systems, typically only effective under 10,000 ft, the guns at much less than that.
They're still useful even today. Medium and long-range SAM's have forced a/c to fly low, below their search and fire control radars, but within range of the guns.
This was standard NATO doctrine during the late Cold War---low level penetration---and gave rise to the F-111 (for example), which was designed to fly fast and low, hugging the bumpy terrain of Central Europe. This would have worked very well had the Soviets not fielded SPAAG's (basically modern AAA) and developed their own MANPADS (shoulder-fired SAM's). Soviet AWACS and look-down shoot-down radar also contributed to the decline of this tactic. These systems denied the use of low-level airspace so effectively that during Desert Storm, the Coalition air forces basically gave up low-level operations within the first week and switched to bombing from medium altitude exclusively (above the reach of AAA and MANPADS). Not even the A-10 with its titanium armor could long survive in that environment. Afterward, they did occasionally venture below 10,000 ft, but typically with special permission and enthusiastic support. Even after the first week, when Iraqi air defenses were ineffective, few a/c still flew below 10,000 ft because the AAA and MANPADS were too numerous to be eliminated entirely.
NATO air forces are getting around this problem by buying stealth fighters, namely the F-35A, which can hide from SAM radars, allowing it to fly at medium/high altitudes, far above MANPADS and AAA. But that's for another question/answer.
Here's an older Shilka (Vitaly V. Kuzmin - http://vitalykuzmin.net/?q=node/306):

A later Tunguska (Leonid Dzhepko / Л.П. Джепко):

And a modern Pantsir (Vitaly V. Kuzmin - vitalykuzmin.net):
(All images taken from their respective wikipedia pages.)

Can a fighter jet traveling at fast speed be brought down by a weapon like an M134 Minigun? or any machine gun?
There are machines guns designed to bring down cruise missiles and jet fighters but they look like this:

see: The Phalanx CIWS close-in weapon system for defense against anti-ship missiles.

I'm pretty sure that's not used to bring down jet fighters -- only their missiles

@Hugh - It works fine on both, assuming they're within range. a/c are often easier to hit, often being larger, slower, and less maneuverable. Phalanx can also target surface vessels, rockets, artillery, and mortars. But, yes, the motivating role is missile self-defense.

@Hugh Both are acceptable targets. (25+ yrs in Navy).  We once had a Learjet towing a target on a long metal cable ... the CIWS hit the cable and the drone lost its motive power.  The pilots (during the debrief) expressed their concern that they were the next likely target since the CIWS didn't acquire the target drone.

@KorvinStarmast my dad (who was an army AA gunner for a while) once told me he and his pals took great pleasure in aiming at the tow cables for the targets rather than the target itself. Harder to hit, and very satisfying seeing that target plummet to the ground rather than just having some holes in it. (this was 1960s, so wooden targets and count the holes rather than modern ones recording rounds passing near).

Phil Handley shot down a MIG19 while his F4 was going supersonic (around Mach 1.2), after both the AIM7 Sparrow and AIM4 Falcon missiles had failed to launch correctly.
Admittedly, this was a one time event, hasn't been repeated since then, but not only can a fast moving aircraft be brought down with a gun, it can be brought down by a gun fired from another fast moving aircraft.

AFAIK the Israelis had quite a few gun kills during their wars. and of course in Korea and possibly even WW2 there were some jets shot down by guns as well.

Virtually all of the Israeli Air Force's air to air victories in the 1967 Six Day War were made using cannon. Some Israeli Mirage III pilots elected not to carry IR guided missiles as they were considered unreliable.

Records suggest that jet fighters are not brought down by hand-aimed bullet-firing weapons.

This article https://sciencebasedlife.wordpress.com/2013/04/03/the-plane-that-shot-itself-down/ demonstrates that at least one jet fighter was shot down by its own machine guns...

training regimen suggests strongly that it was certainly deemed viable into the 1960s.

7.62 mm machine guns are not usually considered adequate for air defence, but are used when nothing better is available.
During the Korean war (1950-53) the .50/12.7 mm projectiles fired from the M3 machine guns of the F86 Sabre proved less effective against the MiG 15 than was desirable, frequently deflecting off without causing damage at shallow impact angles.

If you don't believe me, check it out for yourself.


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Can a helicopter enter Vortex Ring State In Auto Rotation? [see qualifiers]
If you are in a vertical descent [no wind] with rotor at max RPM Auto, and you abruptly pull full collective, the momentum of the blades will drive air down creating down wash.
Now obviously this is a totally hypothetical question with no real world use, but, presuming a high enough inertia rotor system, in this very specific scenario is it possible to at least enter the incipient phase of VRS? [before the rotor folded and you fell to your death]
Please no answers just saying you go into auto to get out of VRS, I am looking for a very specific yes or no.

Seems to be very well answered already here: https://aviation.stackexchange.com/questions/43028/why-is-vortex-state-impossible-in-autorotation?rq=1. Essentially, you cannot get VRS in autorotation, because in autorotation you are falling and leaving the downwash behind, above the rotors.

Autorrotation and VRS are very distinct and incompatible states of the rotor. The answer is no unless you add enough qualifiers to make one of the two states unrecognisable (and thus no longer aptly named)

Are you actually trying to ask "Is it possible to enter VRS from autorotation, without engine power?" to which I suspect this comment may be helpful (although I believe it deserves its own answer here by a more knowledgeable person than me).

That's not what he's asking. He's asking if VRS can be induced in the brief interval where you exit autorotation by pulling pitch, as in the post-flare landing phase, in some hypothetical helicopter that had oodles and oodles of rotor inertia. Not a totally loopy question. A UH-1 can lift off the ground, do a 360 degree pedal turn, and set down again on rotor inertia alone if the rotor rpm is at the top of the green arc when it's started. So the question is how much time does it take to get a VRS going in a vertical descent under power (VRS and SWP are two different things).

Possible duplicate of Why is vortex state impossible in autorotation?

It really is not, VRS is impossible in a state of auto-rotation, but this question is asking in the process of auto rotation, ie. If your descending vertically and apply pitch. the "autorotation" in the question is not the aerodynamic state, but the series of actions taken by the pilot.

To enter VRS you have to have near zero forward airspeed and a descent rate of about 500 fpm or more under power.  Note that VRS is not "settling with power" regardless of what the FAA says - they are two different phenomena.  So in practice you would never encounter the conditions that could create a VRS in the first place while autorotating because you normally autorotate with substantial forward airspeed if you want to be able to land in one piece.
But say if you were autorotating, then slowed to zero airspeed so you were now autorotating straight down (and the descent rate goes from holy crap to oh my god), and then you pulled pitch to arrest the descent rate using rotor inertia. Could a vortex ring state be induced during that brief interval?  Perhaps one could start to form in that brief few seconds of inertia-generated downwash, but by then the rotor would be stalling and you are now an anvil, so the point is moot.
However, for the purposes of our just-for-fun theoretical discussion, I think that if you had enough rotor inertia, it would be possible theoretically I would say, because for the purposes of the time interval involved, it would be the same as having engine power.  Maybe if you had a rotor with super duper uranium tip weights that could generate inertia lift for say 10 seconds or more.
Watch this video to see how fast the vortex ring state can form in a vertical descent with power

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the temperature of intake air at several speed of sound intervals from 1x to 10x?
I watched a YouTube movie about Reaction Engines development of the Sabre engine which main feature is cooling intake air. However, I cannot find an overview of the temperature of intake air at different airspeed levels. I am looking for an overview based on speed of sound levels and the increase on intake air temperatures.

Can you provide link to the youtube movie?

company = http://www.reactionengines.co.uk and youtube = https://www.youtube.com/watch?v=wg2T7MUULZQ

Thermodynamics is your friend here. Compression in a ram intake is ideally isentropic (meaning entropy stays constant, so the whole process is reversible). In reality, some viscous losses cannot be avoided but are in the low percentage range in a well designed intake.
The temperature $T$ in isentropic compression from a state 1 to a state 2 can be calculated from the pressure $p$ and the ratio of specific heats $\kappa$:
$$T_2 = T_1\cdot\left(\frac{p_2}{p_1}\right)^{\frac{\kappa-1}{\kappa}}$$
$\kappa$ is 1.4 for air, so if the saber engine flies in air 250 K cold (-23.15°C) and a pressure of 500 mbar, and the intake compresses this air by a factor of 10 to 5,000 mbar, the temperature will ideally rise to 483 K (210°C). In reality, this might well be 220°C or 230°C due to viscous heating of the air in the process.
How the temperature ratio depends on flight Mach number $Ma$ (again, ideally) can be seen from this plot, based on this way to calculate the pressure rise:


text:
How should I interpret the difference in flight direction (azimuth) for great circle and rhumb line?

I am trying to understand the image above. If you fly in the Northern Hemisphere going west. The image said it's 150 degree if you fly Great Circle while 270 degree flying rhumb line.
If you think about this on a map, how is it possible to be 150 degree? Shouldn't the degree be between north(360) and west(270) as you fly west? I'm so confused.

Welcome. Don't think at it on a map, think at it on a sphere. the heading depends on how the two end points of the path are located related to each other.

It's not just that 150°. Most of the green numbers are wrong, really.

I have no idea where the image comes from, but it seems that you have spotted an inconsistency/copy-paste typo.
Let's remember that the heading is always measured w.r.t. the north, increases in a cloclwise fashion and indicated the direction the aircraft is flying to, as shown in the following figure:

(Image Source: www.allstar.fiu.edu)
On the Rhumb line both East and West directions are shown both with arrows ($\rightarrow \;\; \leftarrow$) and with the initial headings ($90^o \;\; 270^o$)
On the Great Circle lines only the westerly arrows appear and only the starting headings from the West are correct. These seems to have been copied to the East side, but they should have also been augmented by $180^o$, i.e. as you were expecting, you should have $330^o$ for the northern hemisphere (in lieu of the $150^o$) and $210^o$ instead of $30^o$ for the southern.

After some thoughts and ask around, finally figure out the answer to this.
It can be depicted by the image below:

(Image Source: Own Work - Author: tipsywacky)
If you are flying great circle, the angle is measured against the meridian. If it starts from left to right, it will be 30 degree increasing and then reach 150 on the other side and vice versa. Drawing it out on the orange is the key.

Did you make this picture yourself?

Just something I discovered this week: On th PC Version of Google maps, there is a distance measuring tool. While it looks like a ruler and draws straight black lines, what happens when you zoom out and measure Paris - New York? Have fun!

@SentryRaven Yeah I made it myself.  haha.  and sweber thanks for the info.  I am gonna try google earth as well. It's always good to have something being able to give us a better understanding.

and vice versa not really, as when you go fromright to left the angles are larger than 180 (yes, you measure against the meridian, but heading angles are always measured clockwise)

Ah, good call @Federico. I'll keep a note to it.  I guess there are different ways of viewing it.  The image uses the 180 degree benchmark or measure against the equator. Thanks for pointing it out.


text:
Why is the wing of large commercial jet aircraft usually located below the fuselage?
When the wing of an aircraft is located above the fuselage, the aircraft is generally considered more stable.
When the wing of an aircraft is located below the majority of the fuselage, the aircraft is generally considered more acrobatic.
Why then, do large commercial jumbojets generally have their wings located towards the center (from top to bottom) or bottom of the aircraft?  On first glance, this design seems odd since one of their primary goals is stability in flight.

Because there is a thing like being too stable

@ratchetfreak Can you expand on that comment into an answer?  In doing so, can you explain why it works for small aircraft but not larger ones?

There are regulations about how fast an airplane must be able to roll a too stable plane will not be able to do so.

Wing position is secondary for roll speed. Inertia, roll damping and aileron size are the driving factors.

First of all, I do not agree with your assessment: The wing of large commercial aircraft is clearly below the fuselage, to allow the cabin not to become divided by the wing spar. The A380 would have better aerodynamics if the designers had interrupted the lower cabin by the wing spar, but the airlines did not want this at all. Please look at the picture below: I hope you will agree that the wing is clearly below the majority of the fuselage. Why this is done has been answered here and here.

Large commercial aircraft have low wings to stow away their long landing gears. Long gears make it possible to stretch the fuselage and still be able to rotate during take-off. Large high-wing aircraft with their low fuselage position are easier to load and unload, at the price that the fuselage taper has to start shortly aft of the landing gear, so no stretching is possible.
A mid-wing design is common to aerobatic aircraft. For ideal roll handling, all parts should be aligned along a single axis, and doing this complicates fuselage design. Only for aerobatic aircraft the benefits outweigh the disadvantages. See the picture of an Extra 300 below:

A high wing is selected when the gear height should be low for loading and unloading, when the downward field of view of the pilot should not be restricted (think parasol design in WW I), when the wing should have more ground clearance during landing (think gliders) or when the wing is meant to pivot horizontally (see picture below for an example). Note that the combination of a high wing and wing sweep will produce too much sideslip stability and needs to be corrected with anhedral. All three (sweep, wing position and anhedral) change with angle of attack, and the combination will still have too much or too little stability away from the design angle of attack.

As Jan Hudec pointed out, pitch and yaw stability depend mostly on tail size and location, and even roll stability is affected by more than the wing's location. The location is a compromise of many, sometimes diverging parameters, and stability is only a small part of it.

When the wing of an aircraft is located above the fuselage, the aircraft is generally considered more stable.

High-wings increase stability in roll. It does not significantly affect stability in pitch and yaw; for those appropriate tail design is needed.

Why then, do large commercial jumbojets generally have their wings located towards the center of the aircraft (from top to bottom)?

It is more efficient.

On first glance, this design seems odd since one of their primary goals is stability in flight.

No, it isn't.
Most important is stability in pitch, because pitch is related to speed and energy management. But that just needs appropriate horizontal stabilizer and high or low wing does not matter. Stability in yaw is also important, but that is achieved by appropriate vertical stabilizer, again with little difference between high and low wing.
But in roll, the aircraft must not be stable too much. High roll stability provokes dutch roll oscillation and reduces manoeuvrability. So aircraft are designed to be only very slightly stable in roll.
And even for roll stability both low and high wing designs can be relatively easily adjusted for the right amount. Roll stability can be increased by adding dihedral and decreased by adding anhedral as needed.

There are regulations regarding

Pitch Rate
Roll Rate
Yaw Rate

Out of this, the regulations regarding roll rate are as under:

§23.157 Rate of roll.
(a) Takeoff. It must be possible, using a favorable combination of controls, to roll the airplane from a steady 30-degree banked turn through an angle of 60 degrees, so as to reverse the direction of the turn within:

For an airplane of 6,000 pounds or less maximum weight, 5 seconds from initiation of roll; and
For an airplane of over 6,000 pounds maximum weight, W+5001,300 seconds, but not more than 10 seconds, where W is the weight in pounds.

(b) The requirement of paragraph (a) of this section must be met when rolling the airplane in each direction with—

Flaps in the takeoff position;
Landing gear retracted;
For a single-engine airplane, at maximum takeoff power; and for a multiengine airplane with the critical engine inoperative and the propeller in the minimum drag position, and the other engines at maximum takeoff power; and
The airplane trimmed at a speed equal to the greater of 1.2VS1 or 1.1VMC, or as nearly as possible in trim for straight flight.

(c) Approach. It must be possible, using a favorable combination of controls, to roll the airplane from a steady 30-degree banked turn through an angle of 60 degrees, so as to reverse the direction of the turn within:

For an airplane of 6,000 pounds or less maximum weight, 4 seconds from initiation of roll; and
For an airplane of over 6,000 pounds maximum weight, W+2,8002,200 seconds, but not more than 7 seconds, where W is the weight in pounds.

(d) The requirement of paragraph (c) of this section must be met when rolling the airplane in each direction in the following conditions—

Flaps in the landing position(s);
Landing gear extended;
All engines operating at the power for a 3 degree approach; and
The airplane trimmed at VREF.

Hence, a too stable plane cannot achieve such rates at the prescribed mach number, and the control surface deflection. So the wing position is kept so as to meet this requirement. Moreover, if you think that such configuration of the wing would make the airplane inherently unstable, the modern computers on the plane with proper feedback systems take care of such a thing.

Can you clarify W+5001,300 seconds, from section (a)2, and W+2,8002,200 seconds from section (c)2. It appears to me to be poor formatting, either in the source or from the copy/paste, but I'm not certain. I've not been able to make sense of it in any way.

@FreeMan I believe they're meant to read (W + 500) / 1,300 and (W + 2,800) / 2,200 respectively (probably originally printed as fractions). They're consistent with an obvious way the formatting could have been mangled, and they make the functions continuous at the 6000 lb breakpoint.


text:
What are the ways to burn fuel faster in case of emergency without fuel dumping system?
In many cases, the maximum landing weight of an aircraft is lower than the takeoff weight due to the amount of fuel it carries.
While some aircraft have fuel dumping systems, others like the A320 family and Boeing 737 do not, and so have to cycle nearby until they have burned enough fuel. (And in case they have to land immediately, they will simply to, but need further inspection due to the high stress on brakes and landing gears)
My question is: If an aircraft has to cycle to burn fuel, are there measures to burn fuel faster? Lower altitude requires more thrust and so needs more fuel, but are there more ways? Like a slight air brake setting?

It'd (also) be interesting to compare the fuel dump rate and the normal fuel burn rate to see how much useful the dump can be (as indirectly suggested by @aeroalias' answer).

Fly faster, if possible, to run the engines higher. Do it at a lower, less aerodynamic altitude. Put out landing gear, other draggy items.

This question (sorry to say) is sort of moot because:

if you have an urgency to land (e.g. cargo fire), you don't care if you are overweight and you'd land anyway
if you have time to dump fuel, the situation isn't as desperate, and you can easily wait just a bit longer.

The large jets (e.g. Airbus A380, Boeing 777) have a fuel dumping system because, with full fuel, these planes can travel up to 12 hours or more! That is to say, if you don't dump fuel, you might have to fly around all day to get down to your normal landing weight.
Airbus A320 and Boeing 737 are different: they usually fly 1~3 hours route, and may stretch to 6 hours on a full tank. So, to begin with, you need less time to burn fuel. Plus, these (relatively) small planes can land on shorter runways, which means your airport choice will not be as restricted as with a large jet.

Now, as to actually answering your question......
Flaps and gears have speed restrictions. You cannot fly full throttle with these deployed. That being said, if the pilots put the airplane in landing configuration (full flaps + gear down), drag will increase significantly and they will need more thrust to maintain altitude and airspeed.
You can technically achieve that with spoilers, but it's dangerous. Spoilers destroy lift, so what you're really doing is reducing the effectiveness of your wings and compensate that by flying faster and/or increasing your angle of attack.
Then again, if you're desperate, just land right away......

"these planes can travel up to 12 hours or more!" https://xkcd.com/870/

Side comment on your first sentence: you may have an onboard emergency (medica, for example) requiring to land ASAP, but without the risk of landing overweight and damaging the plane.

Ok, old answer, but: there is no such scenario as having to fly all day to get to landing weight. Not even close.

Fuel flow is proportional to air density, so it is advisable to stay low. Since the propulsive power requirement scales with the cube of airspeed, flying faster is also advised. However, when flying below 10.000 ft all civilian traffic has to obey a speed limit of 250 KIAS.
My recommendation is to

stay low and fly in approach configuration (gear, flaps and slats out) as close to 250 KIAS as the configuration allows. A Hapag-Lloyd A310 demonstrated in 2000 how much an extended gear will increase fuel consumption.
climb to an altitude just above FL100 and fly as fast as possible. Depending on the aircraft, spoilers or speed brakes might still be used to increase drag, but the gear and high lift devices have to stay in.
fly a sawtooth pattern: Climb and descent in rapid succession, and use all drag-increasing means during descent, including sideslip.

Which version is best depends on the particular airframe. My personal favorite is number three: Few maneuvers beat a proper sideslip for sheer fun.

I'm going to go out on a limb and say that fun is not high on the priority list when looking for ways to burn fuel due to an in-flight emergency. However, fun can be an added benefit to highly effective. ;)

@FreeMan If it's an emergency haven't you already landed?

@DavidRicherby as per kebs comment on Kevin’s answer, in a medical emergency you don’t want to risk the lives of everyone else by landing over weight.

Speed limit does not apply in an emergency.

Another method - one employed by F-16 aircrews as the jet doesn't have a fuel dump vent - is 'burner and boards', that is selecting afterburning with the speedbrakes deployed.

For increasing fuel burn, the pilot can simply increase drag by increasing the speed. This results in requirement of more thrust, which requires more fuel. He can do it in lower altitudes, but the holding altitude has to be cleared by ATC, I guess.
Note that even jettisoning of fuel takes quite some time.

Can you do something the analog of pressing the gas pedal with the parking brake on?e.g. Add thrust with spoiler + flaps + speed brakes  ON + gear down + RAT deployed or some allowable combination of all those that does not exceed allowable speeds?

Flaps and gears have speed restrictions; flying faster than that can damage them (or even tear them off the aircraft!). RAT won't increase drag significantly.

Many times burning fuel off is not necessary.  However there are times when it is. One pilot who flew for a company I worked for was ferrying a large Cessna from Kansas to Africa, with essentially a waterbed full of avgas.  He had a gear problem at takeoff.  The decision was to empty the bladder inside the cabin, prior to a landing with one gear down and one main someplace else.  Probably a good decision, but this type of situation is rather rare, thankfully.
One human factors consideration is that an extended burn or dump time can increase crew fatigue and stress.That can increase risk.


text:
What is the working principle of anti-explosion fuel tanks?
I recently read a question about Anti Explosion fuel tanks which are self sealing. But the technology seemed to be too old after a bit of research, and that it was difficult to apply this age old technique to pressurized fuel tanks at high altitudes. What are the modern technologies being used for auto - sealing of fuel tanks after being hit by enemy fire to avoid detonation?

Modern military fuel tanks use the same basic technology to seal the tank after a puncture as WW2 aircraft did, which is the use of a laminated rubber-textile material, which tends to retain its form after being torn. The main difference between modern tank liners and WW2 liners is that the modern liners use more sophisticated materials so they can achieve the same benefit using a much thinner liner (hence less weight).
Bullets hitting a tank do not cause "detonation" unless they generate a spark and the fuel-air mixture in the tank is in ignition range. Also, most aircraft explosions are not detonations, they are deflagarations. The liner does nothing to prevent this. The main functions of the liner are to prevent fuel from leaking out of a punctured tank and to maintain the integrity of the tank in a low-altitude mishap or crash. The main test for a liner is the "drop test" in which the liner is filled with fuel and dropped from a height of about 50 feet.
The advantage of pressurizing a tank with air (inerting) is that it increases the critical temperature of an explosive mixture. If the critical temperature of the mixture is above the ambient temperature, then the tank will not explode spontaneously when sparked. A modern variation on this is called "nitrogen inerting". The idea with that is to pressurize the tank with nitrogen. This makes the tank spark proof.
Note that inerting technologies do not prevent a fuel tank from blowing up if an aircraft is hit with ordnance. The reason for this is that typically the way an aircraft blows up is that damage from the ordnance starts a fire. The fire then rapidly evolves fuel-laden gases into open compartments of the aircraft. As soon as these gases reach the ignition mixture, the fire deflagarates the mixture, causing an explosion. Inerting does little to prevent this.
Inerting is very useful, however, because it can prevent a plane blowing up due to lightning or problems with electrical wiring or static electicity buildup.


text:
Why Air France 447 could not reduce angle of attack?
I just finished reading this article and couldn't understand why in 3 minutes and 30 seconds even though there was no reliable speed data, pilot could not level the plane? Isn't the angle of attack reported by a gravity based tool like in off-road vehicles?
Summed up Question: When it's unclear what is happening with the aircraft, isn't it safer to put it in level position and medium speed? (Sorry for rudimentry language here)

In the article it's mentioned that speed had dropped very quickly, isn't it enough indication to at least level the plane which seems to be the safest when you are not clear which direction the plane is heading?

Gravity doesn't work for this, as the load on an aircraft is not always 1G.

But my point is, with sudden loss of speed, you would feel pull forward, which is inertia, not gravity.

I already did and seems like it's all instruments and visual. So I assume inertia/Gs don't really matter at that point in high stress situation. And seems like if you close eyes on a roller coaster, you can't really tell which was you are going.

@egid: I beg to differ - one of the primary flight instruments is the pilot's rear end. It is really easy to feel the difference in attitude between stall and level flight with your botty alone. I for myself have wondered just like Alexus how in hell these people could ignore such obvious indications.

Peter, that's not always true. Accelerated stalls, stalls at very high speed (such as an airliner at cruise, etc) and the like will not have normal feedback.

There was nothing stopping the crew from lowering the nose and flying out of the stall.  In short, they got it all wrong.  The angle of attack has no relationship to gravity.  The angle of attack can be any angle relative to the Earth, and therefore to gravity.

Furthermore, they actually had reliable speed data the entire time that the aircraft was stalled. They just didn't believe it because it was such an unrealisticly low value and the stall warning had stopped indicating a stall because it was so low. There are significant human factors and design issues that led up to this accident.

From reading the BEA report, I believe the biggest problem contributing to the outcome was a failure of CRM. As senior co-pilot, Robert had the opportunity to, for example, use his stick priority button, to direct Bonin to work through the ECAM messages and procedures and by communicating effectively, to bring order to the crew's actions. He didn't do anything like this.

Also, @PeterKämpf, our sensory organs are super easy to fool when vision doesn't help, as in flight in IMC. There's a reason you don't fly by the seat of your pants in the clouds: it will kill you. Tons of illusions exist that make you feel something utterly different from what the airplane is doing. Having students attempt to follow turn/climb/descent instructions while blindfolded can lead to some scary unusual attitudes.

@egid: Agreed, but when the aircraft flies at the same, stalled attitude for several minutes, this is no longer an accelerated stall. This was as stationary as they can get. And that stall at very high speed was still a stall at low dynamic pressure. A very conventional low-speed stall. A completely unnecessary and fully avoidable stall.

Not going to argue that - I just want to be clear that your ass is a tolerable supplementary flight instrument when flying GA in VMC, but that it should not be a datasource pretty much anywhere else.

The reason I was stating relation between gravity and angle of attack is because I thought the TOOL that shows the angle of attack is a simple ball in fluid with markings on it driven by gravity. So that tools is as reliable as it gets.

They couldn't reduce their angle of attack because the first officer was pulling back on the stick the whole time.

I just finished reading this article and couldn't understand why in 3
  minutes and 30 seconds even though there was no reliable speed data,
  pilot could not level the plane?

If you look at this video by the French accident investigation bureau (BEA) you can see what happened.
The problem started when the speed indicators failed temporarily because of icing of the pitot tubes. The aircraft was flying through the intertropical convergence zone (ITCS) where severe thunderstorm activity can build up to high altitudes.
Instead of maintaining attitude and thrust setting, which is a normal procedure when speed indication is lost, the pilot flying (PF) brought the nose up. The aircraft started to climb but at the same time lost a lot of speed (trading kinetic energy for potential energy). Eventually the airspeed dropped so far that the aircraft stalled. By the time the aircraft entered the stall, the pitot tubes where de-iced and all speed indicators were working again. From the moment the aircraft stalled it started to sink. The PF tried to maintain a nose-up attitude, eventually using full aft stick. We can only guess to the reason why; it is likely that he was disoriented and had no confidence in the instruments after the failure. Yet it goes against basic flying instincts to pull the nose up in a stall.
The aircraft is naturally stable and will automatically drop its nose in case of a stall so it took some effort by the PF to keep the nose up. The horizontal tailplane then stalled as well which removed a lot of the nose-down tendency. The result was that the aircraft was in a nose-up attitude for the most part of the descent.
The angle of attack remained therefore too high and the aerodynamic stall persisted all the way down. If the crew would have brought the nose down they would have increased the speed, lowered the angle of attack and recovered from the stall. Once recovered from stall they could then level off the plane.
It may have contributed to the confusion of the crew that the airspeed was below 60 knots during portions of the descent. As they accelerated through 60 knots, the stall warning re-appeared.
The stall indicator systems makes use of alpha vanes which need some airflow over them to work. By inhibiting the stall warning below 60 knots the system is prevent from giving nuisance warnings on the ground. Such a speed is not normally encountered during flight, even not during a stall.
Another contribution to the accident may be that the pilot-not-flying (PNF) cannot see what stick inputs are given by the pilot flying. If it would have been in a different aircraft with coupled yokes, the PNF may have realized what was going on.

Isn't the angle of attack reported by a gravity based tool like in
  off-road vehicles?

The angle of attack is the angle between the chord of the wing and the incoming airflow. It is not related to gravity.

Summed up Question: When it's unclear what is happening with the
  aircraft, isn't it safer to put it in level position and medium speed?
  (Sorry for rudimentry language here)

Initially only the speed indicator failed. If the crew would have remained at the same attitude and power setting the aircraft would not have stalled. That is the standard procedure in such a case ("thrust and attitude")  That may not have been the proper reaction in this case. The aircraft was encountering severe turbulence and the fastest way out may have been to climb. But after the stall warning sounded all speed indicators were back to normal and the aircraft was flyable.

Re conflicting stick inputs: The CVR transcript has six instances of "dual input" announcements. Also Robert earlier warned Bonin numerous times that he was climbing when he'd been told (and agreed) to descend. After Bonin said he had no control anymore and Robert said "controls to the left", Robert evidently did not use the stick priority button on his stick to disable Bonin's inputs when the aircraft warned him six times of dual inputs. CRM failure.

Something I've seen written but not discussed on many of these internet discussions, is the how the stall warning sound worked.
Apparently the stall warning sounded, but stopped again as the plane went to higher angles of attack.  (This might be a deliberate behaviour, to prevent false warning sounds when operating on the ground in wind.)
As a pilot lowered the angle of attack, suddenly the stall sound started again.  One of the pilots may have heard this and instinctively pulled up again to clear the sound.  The angle was kept (very) high for the rest of the flight...
(I'd say the pilots had at least some reason for genuine confusion in this matter.)

Pulling up when you hear a stall warning is the opposite of what you should do, there's no reason the stick should have been pulled back again.

...we all agree, sitting at home in daylight, but in their case, pulling back the stick cleared the stall warning.  The warning behaviour probably contributed to the confusion of the one pilot who was pulling back.  (There's more on the released transcript.)

I've read the transcript, my personal opinion is that there was a lack of fundamental flying skills which comes from operating a computer with wings. The urge to make an auditory warning go away is a product of that. Perhaps flying time without automation should be mandatory to keep pilots familiar with their roots.

@gdd It's easy to look back and say that, but keep in mind that they stalled in IMC conditions, over the water, at night, with what they felt were unreliable instruments.  It was a terrible combination and the deck was definitely stacked against them. Yes, I agree that if they had understood what was happening that it would have been very easy for even many student pilots to fix. Instrument failures, that in real life can happen much differently than in training, can be hard to figure out, especially in the limited time available here. They made mistakes for sure, but not as easy to see.